Three answer off the top of my head.
First, see this section of the user manual about the Time Stretch of audio regions.
http://help.apple.com/logicpro/mac/10.1/?lang=en#/lgcpf7c0ebee
Then this control in the menus.
Third, when you compress or stretch audio regions, you're affecting content, and gain levels will be effected, also. If you don't want this, and/or want better quality stretching, use the Flex tool to retime regions. It is a far superior method, and makes the quick stretch pretty much antique technology.
http://help.apple.com/logicpro/mac/10.1/?lang=en#/lgcp15968647